Heretofore, it has been common that a soundproofing material for the purpose of sound absorption or sound insulation is mounted at a dashboard portion partitioning a vehicle interior from an engine room of an automobile to take a measure to prevent intrusion of vibration or noise from the engine room into the vehicle interior. As such a soundproofing material, one having a sound insulator such as a rubber, polypropylene sheet or vinyl chloride sheet integrated on a sound absorbing material made of a porous material such as polyester fiber or flexible polyurethane foam (hereinafter referred to also as a flexible foam), has been used. Especially, a flexible foam has a merit in that it can be produced inexpensively as compared with a polyester fiber and thus has been used as a sound absorbing material. For example, Patent Document 1 discloses a flexible foam having specific levels of air flowability and hardness, and a process for its production. Further, Patent Document 2 discloses a process for producing a flexible foam which is light in weight and excellent in the soundproofing performance.
However, a conventional flexible foam has been inadequate in the sound-absorbing performance in a low frequency region by itself and thus has had a problem that it is necessary to take a measure to improve the sound insulating property, for example, by bonding to the flexible foam a thermoplastic material such as a rubber containing a polyolefin as the main component. This in turn has led to a further problem that it is required to consider the adhesion between the flexible foam and the thermoplastic material, or that the mass will increase. Patent Document 3 proposes a soundproofing material made of a flexible foam alone, which shows excellent sound absorbing performance in a range of from a medium frequency to high frequency (at least 2,000 Hz), but it has had a problem that the sound absorbing performance is inadequate in a low frequency region of at most 1,000 Hz, particularly at most 500 Hz.
Patent Document 1: JP-B-7-59389
Patent Document 2: JP-A-5-209036
Patent Document 3: JP-A-10-121597